Two major projects have been pursued during the past year: (1) the light scattering photometer described in the 1975/1976 annual report has been improved and used to study the polymerization of hemoglobin S. Control experiments on dilute hemoglobin solutions have also been completed. The kinetic progress curves for the polymerization exhibit a clear delay, but the appearance of significant changes in the scattered intensity can be observed much earlier in the course of the reaction. From these experiments the reaction appears to follow almost a purely autocatlaytic time course through the first three decades of detectable signal change; (2) a study of the polymerization of sickle cell hemoglobin partially liganded with carbon monoxide has been completed. This system provides a model for allosteric control of an assembly reaction by bound ligands and is also relevant to sickling in vivo since the polymerizing hemoglobin in red cells is nearly always partially saturated with ligand. Using linear dichroism measurements on the aligned polymer phase both partially and fully liganded hemoglobin molecules were found to be excluded from the polymer phase. The solubility and kinetic behavior of the partially liganded solutions (up to 70%) can be rationalized by invoking only the excluded volume of the liganded hemoglobin molecules and very limited copolymerization. BIBLIOGRAPHIC REFERENCES: Hofrichter, J., Ross, P.D. and Eaton, W. A.: Supersaturation in sickle cell hemoglobin solutions. Proc. Nat. Acad. Sci. USA 73: 30355-3039, 1976. Hofrichter, J., Ross, P.D. and Eaton, W. A.: A physical description of hemoglobin S gelation. In Hercules, J. I., Cottam, G. L., Waterman, M.R. and Schechter, A. N. (Eds.): Proceedings of the Symposium on Molecular and Cellular Aspects of Sickle Cell Disease. DHEW Publication No. (NIH) 76-1007, National Institutes of Health, Bethesda, Md., 1976, pp. 185-223.